The present invention relates generally to electrical steels, and more specifically, to motor lamination steels having improved magnetic properties in the rolling direction, as well as good mechanical properties.
Desired magnetic properties of steels used for making motor and transformer laminations are low core loss and high permeability. Those steels which are stress relief annealed after punching should have mechanical properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks.
Continuously annealed silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since the steels are substantially free of strain, they can be used in the as-punched condition (commonly referred to as fully processed steels) or can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (commonly referred to as semi-processed steels) to produce the desired magnetic properties with little danger of delamination, warpage, or distortion. Continuous annealing processing requires the electrical steel sheet manufacturer to have a continuous annealing facility. The equipment for a continuous annealing facility requires a capital expenditure of many millions of dollars.
To avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by normal cold rolled sheet processing including batch annealing followed by temper rolling. Continuous annealing processes differ in many respects from normal cold rolled sheet processing. For example, continuous annealing subjects the coil to uniform annealing conditions, whereas batch annealing does not.
In addition, a continuously annealed product does not require temper rolling for flattening, because when steel is continuously annealed it has little strain imparted to it from the annealing process. Although batch annealing facilities use much lower cost equipment than continuous annealing facilities, batch annealing facilities are not able to produce a sufficiently flat product without temper rolling. Strain imparted by temper rolling leads to the delamination and warpage problems of motor lamination steel. At the present time, delamination and warpage resulting from this strain is a serious concern to such customers.
Steel can be produced to have either xe2x80x9corientedxe2x80x9d grains, or xe2x80x9cnon-orientedxe2x80x9d grains. Grain oriented silicon steels are characterized by very light permeability and low core loss in the rolling direction. For example, at 1.5 Tesla (xe2x80x9cTxe2x80x9d) and 60 Hertz (xe2x80x9cHzxe2x80x9d), a 0.012 inch thickness strip may have a permeability in the rolling direction of 28,000 Gauss/Oersted (xe2x80x9cG/Oexe2x80x9d) and a core loss in the rolling direction of 0.58 Watts/pound (xe2x80x9cW/lbxe2x80x9d).
Grain oriented silicon steels have superior magnetic properties in the rolling direction as a result of a so-called Goss texture, i.e., a {110} less than 001 greater than  orientation as defined by the Miller crystallographic indexing system. Steel having a Goss texture is magnetically anisotropic, i.e., it has a sheet-plane variation of permeability and core loss from the rolling direction (0xc2x0) to the transverse direction (90xc2x0). In grain oriented steel, the rolling direction coincides with the easily magnetizable  less than 001 greater than  crystal axes and the grains in the steel occupy a very sharp {110} less than 001 greater than  texture. It is generally believed to be desirable for grain oriented steel to have a substantially complete Goss texture. To this end, an average displacement angle of individual grains from the {100} less than 001 greater than  orientation is as small as possible, for example within 3xc2x0.
A typical process for making grain oriented silicon steel generally includes hot rolling a high alloy steel, containing about 3% or more by weight of silicon. The steel is then solution annealed to dissolve second phase particles and is closely control cooled to produce fine second phase precipitates. Next, there is a two-stage cold reduction, with an intermediate annealing operation. The cold rolled sheets are then primarily recrystallized in a decarburizing atmosphere to remove particles that inhibit grain growth. Secondary recrystallization is then employed in order to grow very large grains ( less than 5 millimeters) possessing the Goss texture. For example, see U.S. Pat. No. 5,342,454 to Hayakawa et al.
One disadvantage of grain oriented silicon steels is that they are expensive to manufacture. Grain oriented steel processing typically requires several costly rolling and annealing steps to produce the Goss texture. Moreover, grain oriented steel processing typically requires the use of a continuous annealing facility.
Another disadvantage of grain oriented steel is that it has poor magnetic properties off-angle from the rolling direction in the plane of the strip. In grain oriented steels, permeability is about 28000 G/Oe in the rolling direction (0xc2x0) and only about 500 G/Oe in the transverse direction (90xc2x0). See the brochure, Armco Oriented Electrical Steels, copyright 1974, Armco Steel Corporation, pages 14 and 36, which is incorporated by reference herein, for typical permeabilities and core losses for grain oriented steel in the rolling direction and off-angle from the rolling direction. Grain oriented steel exhibits a very steep drop in permeability even slightly off-angle from the rolling direction. For example, a typical grain oriented steel has a greater than 50% reduction in permeability between the permeability in the rolling direction and the permeability at 10xc2x0 from the rolling direction.
An inconvenience of using grain oriented steel is that the permeability is so high it may create problems in some devices. For example, transformer light ballast manufacturers have indicated that typical grain oriented material is undesirable in fluorescent light ballasts because it causes a humming sound when the device is operated.
Conventional non-oriented cold rolled sheet processing includes the steps of hot rolling, coiling, pickling, optional hot band annealing, cold rolling, batch annealing and temper rolling. The equipment for such non-oriented processing costs much less than the equipment for a continuous annealing facility. Non-oriented steel processing often employs compositions that desirably have less silicon than grain oriented steel compositions. However, non-oriented steel has a mostly random distribution of orientations. That is, the magnetically xe2x80x9csoftxe2x80x9d  less than 001 greater than  directions occupy a fairly uniform distribution in space, not only in the plane of the sheet but also pointing into and out of the sheet where they participate only minimally in the magnetization process. As a result, non-oriented steel does not exhibit a significant improvement of magnetic properties in the rolling direction.
The present invention utilizes the low-cost attributes of traditional non-oriented processing of cold rolled electrical steels to produce a new class of steel having the Goss texture found in expensive higher alloy grain oriented materials. The steel produced in accordance with the invention has exceptional magnetic properties in the rolling direction, as well as good magnetic properties across a broad range of angles from the rolling direction in the plane of the strip.
Generally, the method of the present invention employs a slab of an electrical steel composition. The composition has up to 2.25% silicon by weight and, in particular, 0.20-2.25% silicon by weight. The composition has up to 0.04% carbon by weight, preferably up to 0.01% carbon by weight. The slab is hot rolled into a strip, which is subjected to steps including hot band annealing in a temperature range effective to coarsen grains sufficient to improve magnetic properties in a rolling direction of the strip, cold rolling, batch annealing in a temperature range effective to produce batch annealed grains of a size not greater than about 40 xcexcm, even more preferably of a size not greater than about 20 xcexcm (corresponding to a temperature preferably ranging from 1040xc2x0-1140xc2x0 F.), and temper rolling with smooth temper rolls. The temper rolls have a smooth surface that is effective to produce a strip with a transfer surface roughness (Ra) of less than 49 xcexcin (wherein xe2x80x9cxcexcxe2x80x9d is the Greek symbol xe2x80x9cmicroxe2x80x9d which means 1xc3x9710xe2x88x926) as well as an increased permeability in the rolling direction after final annealing of preferably at least about 5000 G/Oe. The temper rolls preferably have a smooth surface that is effective to provide the strip with a transfer surface roughness (Ra) of not grater than 15 xcexcin.
More specifically, electrical steel articles are manufactured from the steel strip by steps including punching out motor or transformer shapes from the strip into laminations, which are then stacked and assembled. The laminations are subjected to a final anneal to produce the electrical steel articles of the present invention. However, as used herein, the electrical steel articles of the invention also include electrical steel strip which has been final annealed after temper rolling without punching into shapes and laminating (such as single strip coupons).
Temper rolling is preferably carried out to reduce strip thickness by an amount up to 10%, even more preferably by an amount ranging from 3 to 10%. Temper rolling may be carried out at smaller reduction in thickness when producing steep strip of smaller thicknesses. In this regard, temper rolling reductions in thickness may decrease by about .7% for each 0.01 inch of a reduction in final thickness of the strip.
A preferred method in accordance with the invention for making electrical steel strip for use in the manufacture of electrical steel articles characterized by low core loss and high permeability in the rolling direction, comprises the steps of:
hot rolling a slab of an electrical steel composition into a strip,
hot band annealing in a temperature range effective to coarsen grains sufficient to improve magnetic properties in a rolling direction of the strip,
cold rolling,
batch annealing at a temperature in the range of 1040-1140xc2x0 F., and
temper rolling to provide the strip with a transfer surface roughness (Ra) of not greater than 15 xcexcin.
Electrical steel articles of the present invention manufactured from the steel strip upon a final anneal, have a grain texture including a {110} less than 001 greater than  orientation, and a transfer surface roughness (Ra) of less than 49 xcexcin, preferably not greater than 15 xcexcin. The inventive electrical steel articles preferably have a permeability in the rolling direction of at least 5000 G/Oe, more specifically, a permeability in the rolling direction in the range of 5000-6500 G/Oe. The core loss is preferably not greater than 1.5 W/lb in the rolling direction.
Use of the phase xe2x80x9ctransfer surface roughnessxe2x80x9d herein means the surface roughness of the steel strip that has been acquired by contact between the temper rolls and the steel strip. Reference to xe2x80x9csmoothxe2x80x9d temper rolls herein means rolls that impart to the steel an improved permeability in the rolling direction (e.g., preferably at least 5000 G/Oe) as well as a transfer surface roughness (Ra) of less than 49 xcexcin and preferably, not greater than 15 xcexcin. All angles referred to herein are taken in the plane of the steel articles with respect to the rolling direction, which is at 0xc2x0, and the transverse direction, which is 90xc2x0 from the rolling direction.
More specifically, the steel articles exhibit a change in permeability of about 5% between the permeability in the rolling direction and the permeability at 10xc2x0 from the rolling direction. The permeability is at least 5000 G/Oe across angles ranging from the rolling direction to 18xc2x0 from the rolling direction. The core loss is not greater than 1.5 W/lb across angles ranging from the rolling direction to 25xc2x0 from the rolling direction.
The steel of the present invention has magnetic properties similar to those found in conventional grain oriented steel, and does not suffer from delamination and warpage problems. Moreover, the method of the present invention uses features of non-oriented cold rolled sheet compositions and processing to produce a product have characteristics of a grain oriented product. Therefore, the present method is much more economical than conventional grain oriented steel processing because it does not require a continuous annealing facility, additional rolling steps and higher alloys. In addition, the steel articles produced by the present invention have the desirable properties of high permeability and low core loss in the rolling direction.
One significant way in which the present method differs from grain oriented steel processing is in the final annealing step. In both the present method and grain oriented steel processing, annealing is performed to reduce lamination edge strain from the punching operation. However, when the consumer receives the conventional grain oriented product in it semi-processed form, the material already possess the Goss texture, which was developed at the mill. The microstructure, and hence the magnetic properties in the rolling direction, of conventional grain oriented steel do not change appreciably during the stress relief anneal by the customer. In fact, many customers of grain oriented products do not even perform a stress relief anneal.
In the present invention, the final or stress relief anneal is employed primarily to relieve the strain induced by temper rolling. This is not the purpose of the stress relief anneal of grain oriented material, because typically no temper rolling is conducted during grain oriented steel processing that would impart such strain. Moreover, the Goss texture is not developed in the steel of the present invention until this final anneal, which is usually conducted by the customer.
The present invention is directed to a new class of steel that is not comprised of substantially all Goss texture as is grain oriented steel. The steel of the present invention predominantly includes the Goss texture, but has a broader distribution of the Goss texture than typical grain oriented steel. As a result, the steel articles of the present invention exhibit higher permeabilities across a wider range of angles from the rolling direction than typical grain oriented material. This permits steel articles made according to the present invention to have permeabilities of 5000 G/Oe or more across angles ranging from the rolling direction to 18xc2x0 from the rolling direction. Also, in the present invention the decrease in permeability between the permeability in the rolling direction and the permeability off-angle from the rolling direction is much less than in grain oriented steel. For example, in the present intention the decrease in permeability between the permeability in the rolling direction and the permeability at 10xc2x0 from the rolling direction is about 5%, which is substantially less than in grain oriented materials.
The steel articles of the present invention are suitably used in any products in which good permeability in the rolling direction is desirable, such as in transformers and ballasts. Because the steel articles of the present invention do not have the extremely high permeability in the rolling direction of typical grain oriented materials, they may be used in fluorescent light ballasts without the humming problems of the prior art. Steel articles of the present invention may also be used in motors in view of the significant cost advantage of the present method.
The foregoing and other features and advantages of the invention are illustrated in the accompanying drawings and are described in more detail in the specification and claims that follow.